1. Field of the Invention
This invention relates to methods of and apparatus for correcting errors in binary data, and particularly, but not exclusively, to methods of and apparatus for correcting errors in digital television signals after recording and reproduction.
2. Description of the Prior Art
It is now quite common for analog signals, such as audio or television signals, to be put into digital form, for example using pulse code modulation, for transmission or for recording. It is also quite usual to add some check words to the coded data words before transmission or recording for use on reception or reproduction to detect and correct errors in the received or reproduced data words. Sometimes the check words are derived quite simply as, for example, in the case of simple parity check words and cyclic redundancy check words. However, such simple methods generally imply a relatively low level of error detection or correction capability, or alternatively that a relatively large number of check words are required to achieve a desired level of security against errors. As all such check words are additional to the data words, and therefore in a sense redundant, more sophisticated methods of generating the check words, which result in an improved error detection and correction capability without undue increase in the number of check words required are in use particularly for digital television signals where the amount of data involved means that even without the addition of redundant words, very high bit rates have to be used. Examples of more sophisticated methods which have been used for digital television signals are various so-called b-adjacent codes, of which the Reed-Solomon code is a particular example, and the Bose-Chaudhuri-Hocquenghem code.
A common problem with prior methods of error detection and correction using check words is that if an error occurs in a check word, for example, during transmission or during recording and reproduction, this may mean that error detection and correction using that check word is impossible, or alternatively wrong detection and correction may occur. In the case of a digital television signal either of these eventualities may result in serious deterioration of a reproduced television picture.
Another problem is that while the use of such an error detection and correction method using check words may permit the detection and correction of up to several errors in the block of data words protected by those check words, the method does not provide adequate protection against a burst error involving a substantial sequence of data words and caused, for example, by a tape drop-out.
In our copending UK patent application No. 8222767 (Ser. No. 2 124 806) and No. 8235962 Ser. No. 2 132 393) we have proposed methods and apparatus for dealing with these problems. The present application is concerned with extensions to and modifications of these earlier methods and apparatus to permit still further improvements in the level of correction that can be achieved at least in some circumstances.